1. Field of the Invention
The present invention relates to laminar flow elements, and more particularly to a precision laminar flow element for use in thermal mass flow sensors and flow controllers.
2. Description of the Related Art
Thermal mass flow sensors and flow controllers typically incorporate a sensing tube that has two heated coils (an upstream coil and a downstream coil) wound around the sensing tube in close proximity to one another. Each heated coil is typically made of thin wire that has a characteristic of resistance change as the wire temperature changes. The heated coils direct a constant amount of heat into a gas stream flowing through the sensor tube. As the mass flow rate of the gas stream increases through the sensor tube, the sensor tube carries heat from the upstream coil to the downstream coil. The resulting temperature difference produces an increase in heat to the downstream coil and a corresponding decrease in heat to the upstream coil, thereby changing the resistances of the two coils. This resistance difference is detected via electronic circuitry and produces an output signal proportional to the mass flow rate of the gas stream.
In most commercially available mass flow sensors, the sensing tube is of a small diameter and operates in a linear flow range so long as the gas flow through the sensing tube remains sufficiently low to be in a pure laminar (not turbulent) state. Gas flow rates through most commercially available thermal mass flow sensors have only measured flow rates through the sensing tube to flows of 20 to 30 sccm or ml/min (0.02 to 0.03 liters/minute).
In order to provide thermal mass flow sensors with extended flow ranges beyond 20 to 30 sccm, a common practice has been to divide the incoming gas mass flow into two paths: a sensing element flow path and a bypass flow path. To match the ratio of gas mass flow passing through the bypass flow path with the gas mass flow passing through the sensing element flow path, thermal mass flow sensors have incorporated a laminar flow element (also termed a restriction flow element) so that a linear relationship develops between the sensing element flow path and the bypass flow path.
Laminar flow elements are designed to produce a pressure differential directly proportional to the mass flow rate of a gas stream. A laminar flow element may be a capillary tube having a small diameter, a bundle of such tubes or other tube-based configurations. In the latter type, a flow path is divided into many small passages or channels (typically round or rectangular) to insure that sufficiently developed laminar flow (or nonturbulent flow) exists. A laminar or nonturbulent flow generally refers to the motion of a gas having local velocities and pressures which do not fluctuate randomly. The relationship between the pressure drop and the mass flow rate for laminar flow also depends upon gas viscosity which can vary over large temperature extremes. Laminar flow element designs therefore have placed some temperature constraints and pressure limits on gas in order to define overall accuracy.
The viscosity of gases, however, is essentially independent of pressure between a few hundredths of an atmosphere and several atmospheres until pressures exceed 150 pounds per square inch (psi). In the range of 10 to 50 degrees Celsius, the absolute viscosity of most gases varies only several percent, decreasing as the temperature decreases. So, even though the temperature effects are quite minimal over the above stated temperature and pressure limits, a thermal mass flow sensor design has compensated for such viscosity changes since the sensing element has been a laminar flow capillary tube. As a result, no detectable flow measuring error has resulted from reasonable swings in gas temperature or pressure.
Another approach in laminar flow element design has been to embed multiple capillary tubes in a plastic housing to achieve a capillary type of laminar flow element. This is a costly type of element design. The inside diameter of capillary tubes has varied considerably from one batch to the next. This made it difficult, if not impossible, for one to fabricate seemingly identical capillary tube elements and obtain identical flow versus pressure characteristics. Accordingly, wide variations in flow versus pressure drop characteristics have been attained. This multiple capillary tube approach has been time consuming and costly.
Another type of laminar flow element utilizes plastic "gates" to form gas flow passages. A person installing this laminar flow element has needed to experimentally cut away various "gates" to allow a correct number of passages to be used to obtain the desired flow versus pressure drop characteristics. This type of experimentation has proved to be quite labor intensive and has not provided a uniform laminar flow path around an outside perimeter of the laminar flow element. Some flow measurement error inevitably resulted when the gas flow through the sensing element was compared to the gas flow through the laminar flow element. Flow distribution varied if some gates were removed only at the bottom of the laminar flow element, as compared to removal of gates at the top of the laminar flow element. This non-uniform flow distribution affected the true ratio between the laminar flow bypass shunt and the sensing tube. With flow ratio errors that varied with flow rate, a non-linear characteristic could develop, thereby reducing overall accuracy.
A high flow version of this laminar flow element has typically been supported at each end using support rings. All bypass flow traveled through the many slots in the laminar flow element. Also, some flow inevitably traveled around the outside of the laminar flow element. It has been very difficult, if at all possible, to insure that the laminar flow element was sufficiently centered so that a radial clearance around the outer surface of the laminar flow element remained equal. If the radial clearance was variable from one laminar flow element to another, then a laminar flow element would have a different flow rate versus pressure drop characteristic when compared to other laminar flow element installations. This would likely mean that an installer would spend considerable labor time trying to "tune up" each such laminar flow element installation. Further, it has been found that if the radial clearance varied so as to exceed a certain measurement, problems arose. Laminar flow characteristics were different from the sensing element characteristics, and non-linear behavior resulted. This laminar flow element design would likely be very costly to manufacture and also lack certain precision.
Another approach to designing a laminar flow element has been the laminar flow element described in Baan, U.S. Pat. No. 5,332,005. Multiple machined plates were provided with various slots and a hole to allow a gas stream to pass through in a laminar flow fashion. This design can achieve a high degree of precision but is expensive to manufacture and would be difficult to disassemble for cleaning purposes.
AALBORG Instrument Company has utilized two forms of laminar flow element in its thermal mass flow sensors. The first type has been for relatively low flow rates. This laminar flow element design has depended upon the radial clearance around a round plug that is inserted into a flow block hole in order to establish a desired flow rate versus pressure drop characteristic across the plug. The plug is screwed into an inlet fitting and is cantilevered out from the inlet fitting. In order to maintain laminar flow, it has been necessary for the radial clearance not to exceed roughly 0.045 inches and to be precisely centered with respect to the flow block hole. Manufacturing tolerances have made it commercially impractical to assure perfect radial clearance between the cantilevered plug and the flow block hole. Examination of some AALBORG thermal mass flow sensors has revealed that the radial clearance at the inlet end varies approximately +/-0.020 inches. Due to the cantilever effect, the outlet end of the plug has been found to be either in contact with the flow block hole or roughly 0.045 inches from being at true center with respect to the flow block hole. This variance in design has yielded grossly different results from one laminar flow element to another, making ease of calibration difficult and time consuming.
Since the laminar flow path has been destroyed when the plug is not in the center of the flow block hole, non-linearities have developed, making it difficult to calibrate a flow sensor within its accuracy requirements. The flow path around the plug has not been even, further destroying the ability to obtain good linear results over any large flow range. In practice, this type of laminar flow element for low flow rates has been demonstrated to yield only about a 10-to-1 range of flow within normally accepted commercial tolerances.
For higher flow rates, AALBORG has provided a conduit containing stainless steel mesh or other porous material sandwiched between screen discs. The discs are attached to each other by means of a threaded bolt going through the center of each screen disc. Using the discs and threaded bolt, the steel mesh may be compressed or elongated to accommodate a variety of fluid flow ranges. It appears that the discs are slightly larger in diameter than the flow block hole, thus allowing the laminar flow discs to be fixed into position due to friction. This design has represented a hit or miss approach to achieving a particular flow rate versus pressure drop characteristic. That is, quite likely, an installer added or removed discs until the desired flow rate versus pressure drop characteristic had been achieved. This approach has been very labor intensive and caused unnecessary calibration time. Also, with this approach, it has been found that poor linearity resulted and the overall accuracy of a thermal mass flow sensor was significantly compromised.